State Key Laboratory of Pharmaceutical Biotechnology and MOE Key Laboratory of Model Animals for Disease Study, Model Animal Research Center, Nanjing University, Nanjing 210032.
College of Animal Science and Technology, Nanjing Agricultural University, Nanjing 210095.
J Biol Chem. 2018 Aug 17;293(33):12793-12804. doi: 10.1074/jbc.RA118.003299. Epub 2018 Jun 29.
The polycomb group (PcG) proteins are key epigenetic regulators in stem cell maintenance. PcG proteins have been thought to act through one of two polycomb repressive complexes (PRCs), but more recent biochemical analyses have challenged this model in the identification of noncanonical PRC1 (nc-PRC1) complexes characterized by the presence of Rybp or Yaf2 in place of the canonical Chromobox proteins. However, the biological significance of these nc-PRC1s and the potential mechanisms by which they mediate gene repression are largely unknown. Here, we explore the functional consequences of Yaf2 disruption on stem cell regulation. We show that deletion of Yaf2 results in compromised proliferation and abnormal differentiation of mouse embryonic stem cells (mESCs). Genome-wide profiling indicates Yaf2 functions primarily as a transcriptional repressor, particularly impacting genes associated with ectoderm cell fate in a manner distinct from Rybp. We confirm that Yaf2 assembles into a noncanonical PRC complex, with deletion analysis identifying the region encompassing amino acid residues 102-150 as required for this assembly. Furthermore, we identified serine 166 as a Yaf2 phosphorylation site, and we demonstrate that mutation of this site to alanine (S166A) compromises Ring1B-mediated H2A monoubiquitination and in turn its ability to repress target gene expression. We therefore propose that Yaf2 and its phosphorylation status serve as dual regulators to maintain the pluripotent state in mESCs.
多梳抑制复合物（PcG）蛋白是维持干细胞的关键表观遗传调控因子。PcG 蛋白被认为通过两个多梳抑制复合物（PRC）之一发挥作用，但最近的生化分析对该模型提出了挑战，即在鉴定非典型 PRC1（nc-PRC1）复合物时，特征是存在 Rybp 或 Yaf2 取代典型的 Chromobox 蛋白。然而，这些 nc-PRC1 的生物学意义以及它们介导基因抑制的潜在机制在很大程度上尚不清楚。在这里，我们探讨了 Yaf2 破坏对干细胞调节的功能后果。我们表明，Yaf2 的缺失导致小鼠胚胎干细胞（mESC）的增殖受损和异常分化。全基因组分析表明，Yaf2 主要作为转录抑制剂发挥作用，特别是以不同于 Rybp 的方式影响与外胚层细胞命运相关的基因。我们证实 Yaf2 组装成非典型的 PRC 复合物，缺失分析确定了包含氨基酸残基 102-150 的区域是组装所必需的。此外，我们鉴定出 Yaf2 的丝氨酸 166 是一个磷酸化位点，我们证明该位点突变为丙氨酸（S166A）会损害 Ring1B 介导的 H2A 单泛素化，进而影响其抑制靶基因表达的能力。因此，我们提出 Yaf2 及其磷酸化状态作为双重调节剂，维持 mESC 中的多能状态。
